1. Field of the Invention
The present invention relates to a secondary battery having a separator sandwiched between an anode electrode and a cathode electrode, and more particularly to a secondary battery having increased active surface areas for improved energy density and high battery capacity.
2. Description of the Prior Art
Batteries are commonly used in a variety of applications for conversion of chemical energy into electrical energy. Batteries can be roughly categorized into electro-chemical batteries, fuel cells and solar batteries. Electro-chemical batteries can be sub-categorized into primary batteries and secondary batteries (rechargeable batteries). In recent years, secondary batteries such as lithium ion batteries and nickel metal hydride (NiMH) batteries have been in great demand. This is due to characteristics such as their high voltage, high capacity, and portability. These characteristics make secondary batteries especially suitable for use in electrical applications such as notebook computers, digital cameras, MP3 (Moving Picture Experts Group, audio layer 3) players, and mobile phones.
Generally, a secondary battery includes an anode electrode, a cathode electrode, an ion conducting separator sandwiched between the two electrodes, and an electrolyte. A good example is the lithium ion battery, which has been in commercial use since about 1991 and which has become even more popular in recent times. In the lithium ion battery, a powder of lithium-cobalt oxide (LiCoO2), lithium-nickel oxide (LiNiO2), lithium-manganese oxide (LiMn2O4) or a like cathode active material is mixed with a binder resin and coated on an aluminum plate, which is thus used as the cathode electrode. A powder of carbonaceous active material such as graphite, coke, meso-carbon micro beads, or carbon nanotubes is mixed with a binder resin and coated on a copper plate, which is thus used as the anode electrode. A porous film such as polyethylene, polypropylene or the like is sandwiched between the anode electrode and the cathode electrode, and is used as the ion conducting separator. A non-aqueous solution containing lithium ions is used as the electrolyte.
In order to satisfy the ongoing demand for miniaturization of the aforementioned electrical devices, it is essential to improve the performance of secondary batteries. The energy density of a secondary battery is one of the most important criteria used to evaluate the performance of the secondary battery. A higher energy density means a higher capacity per volume, and a longer continuous working lifetime per recharge. This enables the battery to have a higher capacity and/or a smaller size.
Generally, there are two approaches to increasing the energy density of secondary batteries. The first approach is to develop new active materials for electrodes which have a higher energy capacity. For instance, in a lithium ion battery, employing carbon nanotubes as the cathode active material provides a much higher capacity of lithium ions compared to employing graphite carbon as the cathode active material. However, research activities needed to develop such new materials need much time and money. The second approach is to increase a reactive surface area of the electrodes. For instance, in a certain type of lithium ion battery, the anode electrode and the cathode electrode may be coiled in a spiral shape or overlapped in a parallel relationship, and duly accommodated in a metallic case made from stainless steel or the like. Alternatively, more layers of the anode electrode and the cathode electrode may be employed. However, this latter solution results in an unduly bulky and heavy battery, and is at odds with the demand for miniaturization. Hence, a secondary battery that overcomes the above-described problems and improves on the prior art is desired.